The present invention relates to a reaction vessel having a descending temperature profile and a thermosiphon and its use in water gas shift processes associated with the production of electricity from fuel cells.
Fuel cells are chemical power sources in which electrical power is generated in a chemical reaction. The most common fuel cell is based on the chemical reaction between a reducing agent such as hydrogen and an oxidizing agent such as oxygen. The consumption of these agents is proportional to the power load. Polymers with high protonic conductivities are useful as proton exchange membranes (PEM""s) in fuel cells. Among the earliest PEM""s were sulfonated, crosslinked polystyrenes. More recently, sulfonated fluorocarbon polymers have been considered. Such PEM""s are described in an article entitled, xe2x80x9cNew Hydrocarbon Proton Exchange Membranes Based on Sulfonated Styrene-Ethylene/Butylene-Styrene Triblock Copolymersxe2x80x9d, by G. E. Wnek, J. N. Rider, J. M. Serpico, A. Einset, S. G. Ehrenberg, and L. Raboin presented in the Electrochemical Society Proceedings (1995), Volume 95-23, pages 247 to 251.
Hydrogen is widely produced for chemical and industrial purposes by converting materials such as hydrocarbons and methanol in a reforming process to produce a synthesis gas. Such production usually takes place in large facilities which are rarely turned down in production for even a few days per year. In addition, the operation of the industrial hydrogen production facilities is often integrated with associated facilities to improve the use of energy for the overall complex. Synthesis gas is the name generally given to a gaseous mixture principally comprising carbon monoxide and hydrogen, but also possibly containing carbon dioxide and minor amounts of methane and nitrogen. It is used, or is potentially useful, as feedstock in a variety of large-scale chemical processes, for example: the production of methanol, the production of gasoline boiling range hydrocarbons by the Fischer-Tropsch process, and the production of ammonia.
Processes for the production of synthesis gas are well known and generally comprise steam reforming, autothermal reforming, non-catalytic partial oxidation of light hydrocarbons or non-catalytic partial oxidation of any hydrocarbons. Of these methods, steam reforming is generally used to produce synthesis gas for conversion into ammonia or methanol. In such a process, molecules of hydrocarbons are broken down to produce a hydrogen-rich gas stream. A paper titled xe2x80x9cWill Developing Countries Spur Fuel Cell Surge?xe2x80x9d by Rajinder Singh, which appeared in the March 1999 issue of Chemical Engineering Progress, page 59-66, presents a discussion of the developments of the fuel cell and methods for producing hydrogen for use with fuel cells. The article particularly points out that the partial oxidation process is a fast process permitting small reactors, fast startup, and rapid response to changes in the load, while steam reforming is a slow process requiring a large reactor and long response times, but operates at a high thermal efficiency. The article highlights one hybrid process which combines partial oxidation and steam reforming in a single reaction zone as disclosed in U.S. Pat. No. 4,522,894.
Modifications of the simple steam reforming processes have been proposed to improve the operation of the steam reforming process. In particular, there have been suggestions for improving the energy efficiency of such processes in which the heat available from the products of a secondary reforming step is utilized for other purposes within the synthesis gas production process. For example, processes are described in U.S. Pat. No. 4,479,925 in which heat from the products of a secondary reformer is used to provide heat to a primary reformer.
The reforming reaction is expressed by the following formula:
CH4+2H2Oxe2x86x924H2+CO2
where the reaction in the reformer and the reaction in the shift converter are respectively expressed by the following simplified formulae (1) and (2):
CH4+H2Oxe2x86x92CO+3H2xe2x80x83xe2x80x83(1)
CO+H2Oxe2x86x92H2+CO2xe2x80x83xe2x80x83(2)
In the water gas shift converter which typically follows a reforming step, formula (2) is representative of the major reaction. In the water gas shift converter, at least a portion of the carbon monoxide, which has a deleterious effect on the operation of certain types of fuel cells, particularly PEM fuel cells, is combined with water to shift the equilibrium to produce hydrogen and carbon dioxide. The reaction is generally conducted over a catalyst. Typical water gas shift catalysts include iron-based catalysts such as zinc ferrite (ZnFe2O4), ferric oxide (Fe2O3), magnetite (Fe3O4), chromium oxides, and mixtures such as iron/chromia (90-95% Fe2O3 and 5-10% Cr2O3). Other shift catalysts employed at lower temperatures include copper promoted zinc oxide, copper promoted chromia, and mixtures thereof. The water gas shift reaction is a highly exothermic equilibrium reaction and requires active control of the reactor temperature to produce the desired products.
Others have attempted to integrate the water gas shift reaction into an integrated fuel processor to produce hydrogen for fuel cells. Typically, these efforts have resulted in an apparatus which attempts to integrate the endothermic reforming reaction zone with the exothermic water gas shift reaction zone in a device which disposes these zones adjacent one another. Often these gases are disposed in concentric annular segments of compact devices.
U.S. Pat. No. 4,746,329 discloses a methanol fuel reformer for use in conjunction with fuel cell power plants comprising a plurality of annular chambers which are interconnected in a manner to promote fuel vaporization, reforming, and shift conversion.
U.S. Pat. No. 4,925,456 discloses an apparatus comprising a plurality of double pipe heat exchangers used for primary reforming in a combined primary and secondary reforming process and apparatus which also may contain a carbon monoxide shift catalyst.
U.S. Pat. No. 5,110,559 discloses a shift converter which is surrounded by a shift converter jacket to which reforming off-gas is sent during startup to heat the shift converter and which is switched to cooling during operation of the shift converter.
U.S. Pat. No. 5,458,857 discloses a combined reformer and shift reactor which comprises a cylindrical reforming chamber arranged within and on the axis of a cylindrical vessel. A steam generator is arranged around the reforming chamber and a plurality of shift reactors extend axially, with respect to the vessel, through the steam generator.
U.S. Pat. No. 5,464,606 discloses a method and apparatus for carrying out the shift reactor which employs a two-section reactor. The second section is cooled by indirect heat exchange with cooled effluent from the second section.
WO 97/44123 discloses a shift converter for use with an electrochemical fuel cell for the production of electricity. The shift converter uses an upstream adiabatic zone and a downstream actively cooled zone. The actively cooled zone is cooled by a pressurized water coolant circulated in cooling coils disposed in the actively cooled catalyst zone which boils as it cools the process gas stream.
EPO 0199878A2 discloses an apparatus for use in a pressurized phosphoric acid fuel cell system which incorporates a shift reactor into the reforming reactor to improve heat transfer from the reformer gas into the reforming reaction chamber.
A water gas shift reaction vessel is required to carry out the exothermic water gas shift reaction while cooling the resulting effluent gases from approximately 750xc2x0 to about 100xc2x0 C. This large temperature gradient implies that thermal stresses within the water gas shift reaction vessel will be large. A large temperature gradient can result in severe operating problems, particularly when portions of the reaction vessel have boiling liquid on one side and water vapor on the other side. When this occurs, because the water vapor has a much lower heat transfer coefficient than the boiling liquid, it is not possible to remove a sufficient amount of heat from the water gas shift reacting side. In addition, when the water gas shift reaction vessel is used in conjunction with a fuel cell for the generation of electricity, the operation or generation of electricity may require a variation in the steam supply rate to the reforming reactions. This variation in the demand for steam occurs because the steam flow rate generally is proportional to the amount of hydrocarbon fed to the process. The hydrocarbon feed rate to the reforming process typically is controlled to match the fuel cell demand for hydrogen, which in turn varies with the demand for electricity. For this reason, variations in the level of boiling liquid, which result from a change in the steam demand and are proportional to the steam flow rate, must be avoided.
It is an objective of the present invention to provide a water gas shift reactor to generate steam in a continuous manner to an upstream reforming process.
It is an objective of the present invention to provide an apparatus that minimizes thermal cycling and the resulting damage from thermal stresses to heat exchanger zones and maintains an essentially uniform thermal profile in the water gas shift reactor.
The present invention combines the shift reactor for fuel cell applications with a small-scale thermosiphon steam boiler in a single reaction zone. The reaction zone contains a catalyst zone that comprises high temperature and low temperature shift catalyst. The catalyst zone is in intimate thermal contact with a heat exchange surface that indirectly exchanges heat with water circulating by thermosiphon in a direction which is countercurrent to the passage of reformed gas through the catalyst zone. A fluid reservoir disposed above the catalyst zone facilitates the thermosiphon circulation and stabilizes the generation of steam while surprisingly providing an essentially uniform temperature at the heat exchange surface. The essentially uniform heat exchange surface effectively eliminates thermal cycling during the operation of the thermosiphon shift reactor.
In one embodiment, the thermosiphon shift reactor of the present invention comprises a vertically extended shell defining an interior volume containing a vertical axis. The vertically extended shell has a gas inlet and a gas outlet. The interior volume extends along a vertical length defining a catalyst zone. The catalyst zone comprises a shift catalyst. A jacket sealingly surrounds at least a portion of the length of the vertically extended shell and defines a flow passage between the vertically extended shell and the jacket. A fluid reservoir is located above the catalyst zone in fluid communication with the flow passage at a height effective to provide a thermosiphon circulation of water/steam between the flow passage and the reservoir.